vance over the land. The seabottom exposed to the at- 

 mosphere and water was attacked by these agents and grad- 

 ually carved into the present topography. Just how far the 

 sea retreated and just how high the land stood at the be- 

 ginning of the glacial period we do not know. The fact that 

 the hills and valleys of the mainland extend eastward under 

 the sea seems to indicate that the shore line was to the east 

 under what is now the Atlantic ocean. 



A new factor now appeared and played a decided part 

 in the movements of the crust of New England. Toward 

 the end of the Pliocene Period and in the Pleistocene period 

 the climate of northeastern North America became so cool 

 that the winter snow fall did not entirely melt the following 

 summer. Glaciers developed in the highlands to the north 

 of St. Lawrence and slowly moved southward until they 

 reached the ocean south of what is now Long Island. The 

 weight of this immense ice sheet seems to have depressed 

 the earth's crust, considerably, under the interior of the ice 

 sheet; slightly near the margin where the ice was thin. At 

 the same time the surface of the ocean was lowered as water 

 was removed and locked up in the great glaciers as ice. 



With the return of a warm climate the glacier melted, 

 slow at first, more rapidly as time went on. As the glacier 

 retreated the ocean waters followed, lapping the ice front as 

 it receded. This advance of the sea was in part due to the 

 low stand of the land not yet recovered from the depressive 

 weight of the ice and in part to the return of water to the 

 ocean from the melting ice. At its greatest extent the ocean 

 waters covered all the parts of New England below the pres- 

 ent 700 or 500 foot contour. The portion above water was 

 an island as the sea extended up the Hudson Valley and met 

 the waters of the Gulf of St. Lawrence in the region of Lake 

 Champlain. It was at this time that the high level deltas 

 were built in our valleys. 



The earth, however, is an elastic body and it soon began 

 to recover from the depression due to the weight of the ice. 

 The new upward movement started in the south and in- 

 creased to the northward as the ice retreated. Like the 

 depression, the uplift was greatest where the ice had been 

 thickest. The record of this differential movement is re- 

 corded in tilted shore lines. These must have been formed 

 by a horizontal water surface. Now, however, one end of a 

 strand line may be scores of feet higher than the other. 

 This upward movement in Maine seems to range from 200 

 feet near the coast to about 800 feet on the Northwestern 

 border. 



95 



